In a variety of manufacturing processes, such as those which form multilayer structures, it may be necessary to subject a prescribed portion of a structure to a selective treatment operation, such as polishing or material removal. Where the nature of the process involves a general exposure of the structure to a material modifying agent, such as a wet etch, polishing slurry, etc. in order to remove a particular material, such as a top layer, the material modifying agent may also attack other portions of the structure, modification of the physical characteristics of which may detrimentally affect the intended configuration of the structure.
As an illustration of this problem, consider the case of semiconductor manufacturing processes in which trench isolation technology is employed to form a plurality of dielectrically isolated semiconductor islands that lie atop an insulator (oxide) layer (which, in turn, is supported on an underlying support handle wafer or substrate). As diagrammatically illustrated in FIG. 1, the trench formation may employ a silicon dioxide layer 11 as a mask material (as opposed to using photoresist as the mask material), the oxide `hard` mask layer 11 overlying the top surface 13 of semiconductor islands 15. This `hard` oxide mask is eventually removed by non-selectively exposing the structure of FIG. 1 to a wet oxide etch.
Unfortunately, as shown in FIG. 2, during this surface oxide etch step, a buried oxide layer 21, upon which the trench-patterned islands 15 are disposed, is subjected to attack by the `hard` oxide removal mechanism (the non-selectively applied oxide etch bath). Namely, in the course of removing the mask oxide from the top surface of the semiconductor island, the oxide etch also enters the trenches and attacks the buried oxide layer 21 at the bottom of the trench pattern, thereby undesirably etching this buried oxide in such a manner as to cause undercutting of the sidewalls 23 of the trench pattern, leaving an overhang or `lip` 25 of island material at the bottom of the trench.
When a dielectric (oxide) layer 31 is subsequently formed on the trench sidewalls and the trench is thereafter refilled with material (typically doped or undoped polysilicon) 33, as shown in FIG. 3, a high stress region is induced at the undercut portion of the islands, leading to dislocation-type defects shown by dotted lines 41. These defects propagate along &lt;111&gt; planes in the silicon lattice of the islands. It is believed that a vertical strain is created (normal to the buried oxide surface) by thermal mismatch between the trench refill material 33 or by `bird's beaking` during trench isolation formation of the dielectric (oxide) layer 31. This strain is relieved along the &lt;111&gt; planes, resulting in the lattice dislocations 41.